1. Technical Field of the Invention
The present invention relates generally to an FM-CW radar apparatus which may be employed in anti-collision systems or cruise control systems installed in moving objects such as automotive vehicles and which is designed to transmit a frequency-modulated radar wave and receive a return of the radar wave from a target object to determine the distance to and relative speed of the target object.
2. Background Art
Recently, radars are used in automotive vehicles to measure the distance to and relative speed of an object present ahead of the vehicle. As one of such radars, an FM-CW (frequency modulated-continuous wave) radar is proposed which is designed to transmit a radar wave which is frequency-modulated with a triangular wave to have a frequency increasing and decreasing cyclically, receive a radar return of the transmitted radar wave from a target, and mix the received radar wave with the transmitted one to produce a beat signal. The frequency of the beat signal (referred to as a beat frequency below) is determined using a signal processor in each of ranges wherein the frequency of the transmitted radar wave increases and decreases. The frequency of the beat signal in the range wherein the frequency of the transmitted radar wave increase will be referred to as a rising beat frequency, and that range will be referred to as a modulated frequency rising range. Similarly, the frequency of the beat signal in the range wherein the frequency of the transmitted radar wave decreases will be referred to as a falling beat frequency, and that range will be referred to as a modulated frequency falling range. If the rising beat frequency is defined as fb1, and the falling beat frequency is defined as fb2, the distance D to and relative speed V of a target may be expressed by the equations (A) and (B) below. EQU V=(C/(4.multidot.f0)).multidot.(fb2-fb1) (A) EQU D=(C/(8.multidot..DELTA.F.multidot.fm)).multidot.(fb1+fb2) (B)
where .DELTA.F is a variation in frequency of the transmitted radar wave, f0 is the central frequency of the transmitted radar wave, 1/fm is the time required for one cycle of frequency modulation (i.e., fm is the frequency of the triangular wave used in modulating the transmitted radar wave), and C is the speed of light.
FIGS. 1(a) and 1(c) show frequency relations between a signal T transmitted from the FM-CW radar and a signal R received by the FM-CW radar.
FIG. 1(a) illustrates for the case where a moving object equipped with the FM-CW radar and a target are identical in speed with each other, that is, where the relative speed V of the moving object to the target is zero. Usually, a return of a radar wave from a target undergoes a delay of time the radar wave takes to travel from the radar to the target and back. Thus, the received signal R is, as shown in the drawing, shifted in phase from the transmitted signal T along a time axis so that the rising beat frequency fb1 will be, as shown in FIG. 1(b), equal to the falling beat frequency fb2.
FIG. 1(c) illustrates for the case where a moving object equipped with the FM-CW radar and a target are different in speed from each other, that is, where the relative speed V of the moving object to the target is not zero. In this case, the received signal R is further doppler-shifted in frequency depending upon the relative speed V so that the received signal R is shifted in frequency from the transmitted signal T, which will cause, as shown in FIG. 1(d), the rising beat frequency fb1 to be different from the falling beat frequency fb2.
The use of the above relations between the rising beat frequency fb1 and the falling beat frequency fb2, thus, allows the distance D to and relative speed V of the target to be calculated.
In recent years, techniques for discriminating between a moving object and a stationary object using the FM-CW radar such as ones taught in Japanese Patent First Publication Nos. 7-98375 and 7-191133 are proposed. Such techniques are based on the physical principle that when a subject vehicle is traveling at a speed VB, a stationary object in front of the vehicle is viewed as approaching at the speed VB. For instance, if the direction in which an object approaches a vehicle equipped with the FM-CW radar is defined as a positive direction, and the speed of the vehicle is defined as -VB, the relative speed of a stationary object located ahead of the vehicle may be expressed as VB. The difference between the rising beat frequency fb1 and the falling beat frequency fb2 may, thus, be expressed by the equation (C) below. EQU (fb2-fb1)=(4.multidot.VB.multidot.f0)/C (C)
Analyzing the rising beat frequency fb1 and the falling beat frequency fb2 using the known Fourier transform, the frequency spectrum of a beat signal (referred to as a rising beat signal below) embracing the rising beat frequency fb1 in the modulated frequency rising range wherein a signal transmitted from the FM-CW radar increases in frequency and the frequency spectrum of a beat signal (referred to as a falling beat signal below) embracing the falling beat frequency fb2 in the modulated frequency falling range wherein the transmitted signal decreases in frequency will be ones as shown in FIG. 2(a).
If the speed VB of the vehicle is known, the difference between the falling beat frequency fb2 and the rising beat frequency fb1 may be found as discussed above. Thus, shifting the falling beat signal by a frequency of (fb2-fb1), it will coincide with the rising beat signal, as shown in FIG. 2(b). The use of this fact allows determination of whether the target is a stationary object or a moving object to be made.
However, the above techniques calculate a shift in spectrum or frequency of the beat signal (i.e., the difference between the rising beat frequency fb1 and the falling beat frequency fb2) only based on the fact that when the vehicle is traveling at the speed VB, the stationary object located ahead of the vehicle may be viewed as approaching at the speed of -VB and thus encounter the problems (1), (2), (3), and (4), as discussed below, associated with lack of accuracy with which a stationary object is discriminated from a moving object.
(1) The lag in response rate and measurement error of a vehicle speed sensor causes the accuracy with which the frequency shift of the beat signal is calculated to be decreased. Specifically, the frequency shift may be determined from the speed of the vehicle, but when the speed of the vehicle is being calculated by a computer built in the vehicle for another control, the lag in inter-control communication and/or a filtering operation will cause a shift between the calculated speed and an actual speed of the vehicle to be produced. Additionally, the vehicle speed sensor usually produces an inherent error in output. It is, thus, difficult to determine the frequency shift of the beat signal correctly only based on the speed of the vehicle.
(2) The direction of a radar beam is not considered in calculating the frequency shift of the beat signal, which will cause the accuracy with which the frequency shift of the beat signal is calculated to be decreased. Usually, radars which are so designed as to be oriented out of the direction in which a vehicle travels and which use a beam steering/scanning sensor produce a shift between the direction of apparent movement of a stationary object and the direction of a radar beam in which it is possible to determine the relative speed of the object using the Doppler effect. The wider a radar range is, the greater will be such a shift.
(3) The comparison of the spectrum of the falling beat signal after shifted with the spectrum of the rising beat signal is usually made only using amplitude information such as the levels of peaks and shape of the spectra, which may cause an error in determining of whether the spectrum of the falling beat signal coincides with that of the rising beat signal or not. Specifically, if there are rising and falling beat signals which are produced by a moving object and which have the same peak level as that produced by a stationary object, it may cause the moving object to be identified as the stationary object in error.
(4) If the peaks of the spectra of beat signals produced by stationary and moving objects are the same, it may cause the peak of the spectrum of the beat signal arising from the moving object to be eliminated. For instance, when the FM-CW radar is used in an automotive vehicle, the spectra of beat signals produced by a stationary object such as a guard rail and a moving object such as a vehicle traveling ahead may be derived at the same time. In this case, the peaks of both the spectra may be combined together depending upon the distance to the stationary object, the distance to the moving object, and the relative speeds of the vehicle to the stationary and moving objects. Particularly, a stationary object having a higher reflectivity such as an entrance of a tunnel will produce a great peak in the spectrum of a beat signal. The frequency near that peak is also wide, which may hide the peak in the spectrum of a beat signal produced by a moving object so that the two peaks may be observed as a single peak.